Radiation melt treated ultra high molecular weight polyethylene prosthetic devices

ABSTRACT

A medical prosthesis for use within the body which is formed of radiation treated ultra high molecular weight polyethylene having substantially no detectable free radicals, is described. Preferred prostheses exhibit reduced production of particles from the prosthesis during wear of the prosthesis, and are substantially oxidation resistant. Methods of manufacture of such devices and material used therein are also provided.

This application is a continuation-in-part of application Ser. No.08/726,313, filed on Oct. 2, 1996, entitled RADIATION AND MELT TREATEDULTRA HIGH MOLECULAR WEIGHT POLYETHYLENE PROSTHETIC DEVICES, which is acontinuation-in-part of application Ser. No. 08/600,744, filed on Feb.13, 1996, entitled MELT-IRRADIATED ULTRA HIGH MOLECULAR WEIGHTPOLYETHYLENE PROSTHETIC DEVICES. The entire contents of the parentapplications are expressly incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to the orthopedic field and the provisionof prostheses, such as hip and knee implants, as well as methods ofmanufacture of such devices and material used therein.

BACKGROUND OF THE INVENTION

The use of synthetic polymers, e.g., ultra high molecular weightpolyethylene, with metallic alloys has revolutionized the field ofprosthetic implants, e.g., their use in total joint replacements for thehip or knee. Wear of the synthetic polymer against the metal of thearticulation, however, can result in severe adverse effects whichpredominantly manifest after several years. Various studies haveconcluded that such wear can lead to the liberation of ultrafineparticles of polyethylene into the periprosthetic tissues. It has beensuggested that the abrasion stretches the chain folded crystallites toform anisotropic fibrillar structures at the articulating surface. Thestretched-out fibrils can then rupture, leading to production ofsubmicron sized particles. In response to the progressive ingress ofthese polyethylene particles between the prosthesis and bone,macrophage-induced resorption of the periprosthetic bone is initiated.The macrophage, often being unable to digest these polyethyleneparticles, synthesize and release large numbers of cytokines and growthfactors which can ultimately result in bone resorption by osteoclastsand monocytes. This osteolysis can contribute to mechanical loosening ofthe prosthesis components, thereby sometimes requiring revision surgerywith its concomitant problems.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an implantable prosthesisdevice formed at least in part of radiation treated ultra high molecularweight polyethylene (UHMWPE) having no detectable free radicals, so asto reduce production of fine particles from the prosthesis during wearof the prosthesis.

It is another object of the invention to reduce osteolysis andinflammatory reactions resulting from prosthesis implants.

It is yet another object of the invention to provide a prosthesis whichcan remain implanted within a person for prolonged periods of time.

It is yet another object of the invention to provide improved UHMWPEwhich can be used in the prostheses of the preceding objects and/or inother fabricated articles.

Still another object of the invention is to provide improved UHMWPEwhich has a high density of cross-links and no detectable free radicals.

A still further object of the invention is to provide improved UHMWPEwhich has improved wear resistance.

According to the invention, a medical prosthesis for use within the bodywhich is formed of radiation treated ultra high molecular weightpolyethylene (UHMWPE) having substantially no detectable free radicals,is provided. The radiation can be, e.g., gamma or electron radiation.The UHMWPE has a cross-linked structure. Preferably, the UHMWPE issubstantially not oxidized and is substantially oxidation resistant.Variations include, e.g., the UHMWPE having three melting peaks, twomelting peaks or one melting peak. In certain embodiments, the UHMWPEhas a polymeric structure with less than about 50% crystallinity, lessthan about 290 Å lamellar thickness and less than about 940 MPa tensileelastic modulus, so as to reduce production of fine particles from theprosthesis during wear of the prosthesis. Part of the prosthesis can be,e.g., in the form of a cup or tray shaped article having a load bearingsurface made of this UHMWPE. This load bearing surface can be in contactwith a second part of the prosthesis having a mating load bearingsurface of a metallic or ceramic material.

Another aspect of the invention is radiation treated UHMWPE havingsubstantially no detectable free radicals. This UHMWPE has across-linked structure. Preferably, this UHMWPE is substantially notoxidized and is substantially oxidation resistant. Variations include,e.g., the UHMWPE having three melting peaks, two melting peaks or onemelting peak.

Other aspects of the invention are fabricated articles, e.g., with aload bearing surface, and wear resistant coatings, made from suchUHMWPE. One embodiment is where the fabricated article is in the form ofa bar stock which is capable of being shaped into articles byconventional methods, e.g., machining.

Yet another aspect of the invention includes a method for making across-linked UHMWPE having substantially no detectable free radicals.Conventional UHMWPE having polymeric chains is provided. This UHMWPE isirradiated so as to cross-link said polymeric chains. The UHMWPE isheated above the melting temperature of the UHMWPE so that there aresubstantially no detectable free radicals in the UHMWPE. The UHMWPE isthen cooled to room temperature. In certain embodiments, the cooledUHMWPE is machined and/or sterilized.

One preferred embodiment of this method is called CIR-SM, i.e., coldirradiation and subsequent melting. The UHMWPE that is provided is atroom temperature or below room temperature.

Another preferred embodiment of this method is called WIR-SM, i.e., warmirradiation and subsequent melting. The UHMWPE that is provided ispre-heated to a temperature below the melting temperature of the UHMWPE.

Another preferred embodiment of this method is called WIR-AM, i.e., warmirradiation and adiabatic melting. In this embodiment, the UHMWPE thatis provided is pre-heated to a temperature below the melting temperatureof the UHMWPE, preferably between about 100° C. to below the meltingtemperature of the UHMWPE. Preferably, the UHMWPE is in an insulatingmaterial so as to reduce heat loss from the UHMWPE during processing.The pre-heated UHMWPE is then irradiated to a high enough total dose andat a fast enough dose rate so as to generate enough heat in the polymerto melt substantially all the crystals in the material and thus ensureelimination of substantially all detectable free radicals generated by,e.g., the irradiating step. It is preferred that the irradiating stepuse electron irradiation so as to generate such adiabatic heating.

Another aspect of this invention is the product made in accordance withthe above described method.

Yet another aspect of this invention, called MIR, i.e., meltirradiation, is a method for making crosslinked UHMWPE. ConventionalUHMWPE is provided. Preferably, the UHMWPE is surrounded with an inertmaterial that is substantially free of oxygen. The UHMWPE is heatedabove the melting temperature of the UHMWPE so as to completely melt allcrystalline structure. The heated UHMWPE is irradiated, and theirradiated UHMWPE is cooled to about 25° C.

In an embodiment of MIR, highly entangled and crosslinked UHMWPE ismade. Conventional UHMWPE is provided. Preferably, the UHMWPE issurrounded with an inert material that is substantially free of oxygen.The UHMWPE is heated above the melting temperature of the UHMWPE for atime sufficient to enable the formation of entangled polymer chains inthe UHMWPE. The heated UHMWPE is irradiated so as to trap the polymerchains in the entangled state, and the irradiated UHMWPE is cooled toabout 25° C.

The invention also features a method of making a medical prosthesis fromradiation treated UHMWPE having substantially no detectable freeradicals, the prosthesis resulting in reduced production of particlesfrom the prosthesis during wear of the prosthesis. Radiation treatedUHMWPE having no detectable free radicals is provided. A medicalprosthesis is formed from this UHMWPE so as to reduce production ofparticles from the prosthesis during wear of the prosthesis, the UHMWPEforming a load bearing surface of the prosthesis. Formation of theprosthesis can be accomplished by standard procedures known to thoseskilled in the art, e.g., machining.

Also provided in this invention is a method of treating a body in needof a medical prosthesis. A shaped prosthesis formed of radiation treatedUHMWPE having substantially no detectable free radicals is provided. Theprosthesis is applied to the body in need of the prosthesis. Theprosthesis reduces production of particles from the prosthesis duringwear of the prosthesis. In preferred embodiments, the UHMWPE forms aload bearing surface of the prosthesis.

The above and other objects, features and advantages of the presentinvention will be better understood from the following specificationwhen read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view through the center of a medical hipjoint prosthesis in accordance with a preferred embodiment of thisinvention;

FIG. 2 is a side view of an acetabular cup liner as shown in FIG. 1;

FIG. 3 is a cross-sectional view through line 3-3 of FIG. 2;

FIG. 4 is a graph showing the crystallinity and melting point ofmelt-irradiated UHMWPE at different irradiation doses;

FIG. 5 is an environmental scanning electron micrograph of an etchedsurface of conventional UHMWPE showing its crystalline structure;

FIG. 6 is an environmental scanning electron micrograph of an etchedsurface of melt-irradiated UHMWPE showing its crystalline structure atapproximately the same magnification as in FIG. 5; and

FIG. 7 is a graph showing the crystallinity and melting point atdifferent depths of a melt-irradiated UHMWPE cup.

FIG. 8 is a graph showing DSC melting endotherms for Hoechst-CelaneseGUR 4050 UHMWPE prepared using warm irradiation and partial adiabaticmelting (WIR-AM), with and without subsequent heating.

FIG. 9 is a graph showing DSC melting endotherms for Hoechst-CelaneseGUR 1050 UHMWPE prepared using warm irradiation and partial adiabaticmelting (WIR-AM), with and without subsequent heating.

FIG. 10 is a graph showing adiabatic heating of UHMWPE treated by WIR-AMwith a pre-heat temperature of 130° C.

FIG. 11 is a graph showing tensile deformation behavior of unirradiatedUHMWPE, CIR-SM treated UHMWPE, and WIR-AM treated UHMWPE.

DETAILED DESCRIPTION

This invention provides a medical prosthesis for use within the bodywhich is formed of radiation treated ultra high molecular weightpolyethylene (UHMWPE) which has substantially no detectable freeradicals.

A medical prosthesis in the form of a hip joint prosthesis is generallyillustrated at 10 in FIG. 1. The prosthesis shown has a conventionalball head 14 connected by a neck portion to a stem 15 which is mountedby conventional cement 17 to the femur 16. The ball head can be ofconventional design and formed of stainless steel or other alloys asknown in the art. The radius of the ball head closely conforms to theinner cup radius of an acetabular cup 12 which can be mounted in cement13 directly to the pelvis 11. Alternatively, a metallic acetabular shellcan be cemented to the pelvis 11 and the acetabular cup 12 can form acoating or liner connected to the metallic acetabular shell by means asare known in the art.

The specific form of the prosthesis can vary greatly as known in theart. Many hip joint constructions are known and other prostheses such asknee joints, shoulder joints, ankle joints, elbow joints and fingerjoints are known. All such prior art prostheses can be benefited bymaking at least one load bearing surface of such prosthesis of a highmolecular weight polyethylene material in accordance with thisinvention. Such load bearing surfaces can be in the form of layers,linings or actual whole devices as shown in FIG. 1. In all cases, it ispreferred that the load bearing surface act in conjunction with ametallic or ceramic mating member of the prosthesis so that a slidingsurface is formed therebetween. Such sliding surfaces are subject tobreakdown of the polyethylene as known in the prior art. Such breakdowncan be greatly diminished by use of the materials of the presentinvention.

FIG. 2 shows the acetabular cup 12 in the form of a half hollowball-shaped device better seen in the cross-section of FIG. 3. Aspreviously described, the outer surface 20 of the acetabular cup neednot be circular or hemispherical but can be square or of anyconfiguration to be adhered directly to the pelvis or to the pelvisthrough a metallic shell as known in the art. The radius of theacetabular cup shown at 21 in FIG. 3 of the preferred embodiment rangesfrom about 20 mm to about 35 mm. The thickness of the acetabular cupfrom its generally hemispherical hollow portion to the outer surface 20is preferably about 8 mm. The outer radius is preferably in the order ofabout 20 mm to about 35 mm.

In some cases, the ball joint can be made of the UHMWPE of thisinvention and the acetabular cup formed of metal, although it ispreferred to make the acetabular cup or acetabular cup liner of UHMWPEto mate with the metallic ball. The particular method of attachment ofthe components of the prosthesis to the bones of the body can varygreatly as known in the art.

The medical prosthesis of this invention is meant to include wholeprosthetic devices or portions thereof, e.g., a component, layer orlining. The medical prosthesis includes, e.g., orthopedic joint and bonereplacement parts, e.g., hip, knee, shoulder, elbow, ankle or fingerreplacements. The prosthesis can be in the form, e.g., of a cup or trayshaped article which has a load bearing surface. Other forms known tothose skilled in the art are also included in the invention. Medicalprostheses are also meant to include any wearing surface of aprosthesis, e.g., a coating on a surface of a prosthesis in which theprosthesis is made from a material other than the UHMWPE of thisinvention.

The prostheses of this invention are useful for contact with metalcontaining parts formed of, e.g., cobalt chromium alloy, stainlesssteel, titanium alloy or nickel cobalt alloy, or with ceramic containingparts. For example, a hip joint is constructed in which a cup shapedarticle having an inner radius of 25 mm, is contacted with a metal ballhaving an outer radius of 25 mm, so as to closely mate with the cupshaped article. The load bearing surface of the cup shaped article ofthis example is made from the UHMWPE of this invention, preferablyhaving a thickness of at least about 1 mm, more preferably having athickness of at least about 2 mm, more preferably having a thickness ofat least about {fraction (1/4)} inch, and more preferably yet having athickness of at least about {fraction (1/3)} inch.

The prostheses can have any standard known form, shape, orconfiguration, or be a custom design, but have at least one load bearingsurface of UHMWPE of this invention.

The prostheses of this invention are non-toxic to humans. They are notsubject to deterioration by normal body constituents, e.g., blood orinterstitial fluids. They are capable of being sterilized by standardmeans, including, e.g., heat or ethylene oxide.

By UHMWPE is meant linear non-branched chains of ethylene that havemolecular weights in excess of about 500,000, preferably above about1,000,000, and more preferably above about 2,000,000. Often themolecular weights can be at least as high as about 8,000,000. By initialaverage molecular weight is meant the average molecular weight of theUHMWPE starting material, prior to any irradiation.

Conventional UHMWPE is standardly generated by Ziegler-Natta catalysis,and as the polymer chains are generated from the surface catalytic site,they crystallize, and interlock as chain folded crystals. Examples ofknown UHMWPE powders include Hifax Grade 1900 polyethylene (obtainedfrom Montell, Wilmington, Del.), having a molecular weight of about 2million g/mol and not containing any calcium stearate; GUR 4150, alsoknown as GUR 415, (obtained from Hoescht Celanese Corp., Houston, Tex.),having a molecular weight of about 4-5 million g/mol and containing 500ppm of calcium stearate; GUR 4050 (obtained from Hoescht Celanese Corp.,Houston, Tex.), having a molecular weight of about 4-5 million g/mol andnot containing any calcium stearate; GUR 4120 (obtained from HoeschtCelanese Corp., Houston, Tex.), having a molecular weight of about 2million g/mol and containing 500 ppm of calcium stearate; GUR 4020(obtained from Hoescht Celanese Corp., Houston, Tex.), having amolecular weight of about 2 million g/mol and not containing any calciumstearate; GUR 1050 (obtained from Hoescht Celanese Corp., Germany),having a molecular weight of about 4-5 million g/mol and not containingany calcium stearate; GUR 1150 (obtained from Hoescht Celanese Corp.,Germany), having a molecular weight of about 4-5 million g/mol andcontaining 500 ppm of calcium stearate; GUR 1020 (obtained from HoeschtCelanese Corp., Germany), having a molecular weight of about 2 milliong/mol and not containing any calcium stearate; and GUR 1120 (obtainedfrom Hoescht Celanese Corp., Germany), having a molecular weight ofabout 2 million g/mol and containing 500 ppm of calcium stearate.Preferred UHMWPEs for medical applications are GUR 4150, GUR 1050 andGUR 1020. By resin is meant powder.

UHMWPE powder can be consolidated using a variety of differenttechniques, e.g., ram extrusion, compression molding or directcompression molding. In ram extrusion, the UHMWPE powder is pressurizedthrough a heated barrel whereby it is consolidated into a rod stock,i.e., bar stock (can be obtained, e.g., from Westlake Plastics, Lenni,Pa.). In compression molding, the UHMWPE powder is consolidated underhigh pressure into a mold (can be obtained, e.g., from Poly-Hi Solidur,Fort Wayne, Ind., or Perplas, Stanmore, U.K.). The shape of the mold canbe, e.g., a thick sheet. Direct compression molding is preferably usedto manufacture net shaped products, e.g., acetabular components ortibial knee inserts (can be obtained, e.g., from Zimmer, Inc., Warsaw,Ind.). In this technique, the UHMWPE powder is compressed directly intothe final shape. “Hockey pucks”, or pucks, are generally machined fromram extruded bar stock or from a compression molded sheet.

By radiation treated UHMWPE is meant UHMWPE which has been treated withradiation, e.g., gamma radiation or electron radiation, so as to inducecross-links between the polymeric chains of the UHMWPE.

By substantially no detectable free radicals is meant substantially nofree radicals as measured by electron paramagnetic resonance, asdescribed in Jahan et al., J. Biomedical Materials Research 25: 1005(1991). Free radicals include, e.g., unsaturated trans-vinylene freeradicals. UHMWPE that has been irradiated below its melting point withionizing radiation contains cross-links as well as long-lived trappedfree radicals. These free radicals react with oxygen over the long-termand result in the embrittlement of the UHMWPE through oxidativedegradation. An advantage of the UHMWPE and medical prostheses of thisinvention is that radiation treated UHMWPE is used which has nodetectable free radicals. The free radicals can be eliminated by anymethod which gives this result, e.g., by heating the UHMWPE above itsmelting point such that substantially no residual crystalline structureremains. By eliminating the crystalline structure, the free radicals areable to recombine and thus are eliminated.

The UHMWPE which is used in this invention has a cross-linked structure.An advantage of having a cross-linked structure is that it will reduceproduction of particles from the prosthesis during wear of theprosthesis.

It is preferred that the UHMWPE be substantially not oxidized. Bysubstantially not oxidized is meant that the ratio of the area under thecarbonyl peak at 1740 cm⁻¹ in the FTIR spectra to the area under thepeak at 1460 cm⁻¹ in the FTIR spectra of the cross-linked sample be ofthe same order of magnitude as the ratio for the sample beforecross-linking.

It is preferred that the UHMWPE be substantially oxidation resistant. Bysubstantially oxidation resistant is meant that it remains substantiallynot oxidized for at least about 10 years. Preferably, it remainssubstantially not oxidized for at least about 20 years, more preferablyfor at least about 30 years, more preferably yet for at least about 40years, and most preferably for the entire lifetime of the patient.

In certain embodiments, the UHMWPE has three melting peaks. The firstmelting peak preferably is about 105° C. to about 120° C., morepreferably is about 110° C. to about 120° C., and most preferably isabout 118° C. The second melting peak preferably is about 125° C. toabout 140° C., more preferably is about 130° C. to about 140° C., morepreferably yet is about 135° C., and most preferably is about 137° C.The third melting peak preferably is about 140° C. to about 150° C.,more preferably is about 140° C. to about 145° C., and most preferablyis about 144° C. In certain embodiments, the UHMWPE has two meltingpeaks. The first melting peak preferably is about 105° C. to about 120°C., more preferably is about 110° C. to about 120° C., and mostpreferably is about 118° C. The second melting peak preferably is about125° C. to about 140° C., more preferably is about 130° C. to about 140°C., more preferably yet is about 135° C., and most preferably is about137° C. In certain embodiments, the UHMWPE has one melting peak. Themelting peak preferably is about 125° C. to about 140° C., morepreferably is about 130° C. to about 140° C., more preferably yet isabout 135° C., and most preferably is about 137° C. Preferably, theUHMWPE has two melting peaks. The number of melting peaks is determinedby differential scanning calorimetry (DSC) at a heating rate of 10°C./min.

The polymeric structure of the UHMWPE used in the prostheses of thisinvention results in the reduction of production of UHMWPE particlesfrom the prosthesis during wear of the prosthesis. As a result of thelimited number of particles being shed into the body, the prosthesisexhibits longer implant life. Preferably, the prosthesis can remainimplanted in the body for at least 10 years, more preferably for atleast 20 years and most preferably for the entire lifetime of thepatient.

The invention also includes other fabricated articles made fromradiation treated UHMWPE having substantially no detectable freeradicals. Preferably, the UHMWPE which is used for making the fabricatedarticles has a cross-linked structure. Preferably, the UHMWPE issubstantially oxidation resistant. In certain embodiments, the UHMWPEhas three melting peaks. In certain embodiments, the UHMWPE has twomelting peaks. In certain embodiments, the UHMWPE has one melting peak.Preferably, the UHMWPE has two melting peaks. The fabricated articlesinclude shaped articles and unshaped articles, including, e.g., machinedobjects, e.g., cups, gears, nuts, sled runners, bolts, fasteners,cables, pipes and the like, and bar stock, films, cylindrical bars,sheeting, panels, and fibers. Shaped articles can be made, e.g., bymachining. The fabricated article can be, e.g., in the form of a barstock which is capable of being shaped into a second article bymachining. The fabricated articles are particularly suitable for loadbearing applications, e.g., high wear resistance applications, e.g., asa load bearing surface, e.g., an articulating surface, and as metalreplacement articles. Thin films or sheets of the UHMWPE of thisinvention can also be attached, e.g., with glue, onto supportingsurfaces, and thus used as a wear resistant load bearing surface.

The invention also includes radiation treated UHMWPE which hassubstantially no detectable free radicals. The UHMWPE has a cross-linkedstructure. Preferably, the UHMWPE is substantially not oxidized and issubstantially oxidation resistant. In certain embodiments, the UHMWPEhas three melting peaks. In certain embodiments, the UHMWPE has twomelting peaks. In certain embodiments, the UHMWPE has one melting peak.Preferably, the UHMWPE has two melting peaks. Depending upon theparticular processing used to make the UHMWPE, certain impurities may bepresent in the UHMWPE of this invention, including, e.g., calciumstearate, mold release agents, extenders, anti-oxidants and/or otherconventional additives to polyethylene polymers.

The invention also provides a method for making cross-linked UHMWPEhaving substantially no detectable free radicals. Preferably, thisUHMWPE is for use as a load bearing article with high wear resistance.Conventional UHMWPE having polymeric chains is provided. Theconventional UHMWPE can be in the form of, e.g., a bar stock, a shapedbar stock, e.g., a puck, a coating, or a fabricated article, e.g., a cupor tray shaped article for use in a medical prosthesis. By conventionalUHMWPE is meant commercially available high density (linear)polyethylene of molecular weights greater than about 500,000.Preferably, the UHMWPE starting material has an average molecular weightof greater than about 2 million. By initial average molecular weight ismeant the average molecular weight of the UHMWPE starting material,prior to any irradiation. The UHMWPE is irradiated so as to cross-linkthe polymeric chains. The irradiation can be done in an inert ornon-inert environment. Preferably, the irradiation is done in anon-inert environment, e.g., air. The irradiated UHMWPE is heated abovethe melting temperature of the UHMWPE so that there are substantially nodetectable free radicals in the UHMWPE. The heated UHMWPE is then cooledto room temperature. Preferably, the cooling step is at a rate greaterthan about 0.1° C./minute. Optionally, the cooled UHMWPE can bemachined. For example, if any oxidation of the UHMWPE occurred duringthe irradiating step, it can be machined away if desired, by any methodknown to those skilled in the art. And optionally, the cooled UHMWPE, orthe machined UHMWPE, can be sterilized by any method known to thoseskilled in the art.

One preferred embodiment of this method is called CIR-SM, i.e., coldirradiation and subsequent melting. In this embodiment, the UHMWPE thatis provided is at room temperature or below room temperature.Preferably, it is about 20° C. Irradiation of the UHMWPE can be with,e.g., gamma irradiation or electron irradiation. In general, gammairradiation gives a high penetration depth but takes a longer time,resulting in the possibility of more in-depth oxidation. In general,electron irradiation gives more limited penetration depths but takes ashorter time, and the possibility of extensive oxidation is reduced. Theirradiation is done so as to cross-link the polymeric chains. Theirradiation dose can be varied to control the degree of cross-linkingand crystallinity in the final UHMWPE product. Preferably, the totalabsorbed dose of the irradiation is about 0.5 to about 1,000 Mrad, morepreferably about 1 to about 100 Mrad, more preferably yet about 4 toabout 30 Mrad, more preferably yet about 20 Mrad, and most preferablyabout 15 Mrad. Preferably, a dose rate is used that does not generateenough heat to melt the UHMWPE. If gamma irradiation is used, thepreferred dose rate is about 0.05 to about 0.2 Mrad/minute. If electronirradiation is used, preferably the dose rate is about 0.05 to about3,000 Mrad/minute, more preferably about 0.05 to about 5 Mrad/minute,and most preferably about 0.05 to about 0.2 Mrad/minute. The dose ratein electron irradiation is determined by the following parameters: (1)the power of the accelerator in kW, (ii) the conveyor speed, (iii) thedistance between the surface of the irradiated specimen and the scanhorn, and (iv) the scan width. The dose rate at an e-beam facility isoften measured in Mrads per pass under the rastering e-beam. The doserates indicated herein as Mrad/minute can be converted to Mrad/pass byusing the following equation:D _(Mrad/min) =D _(Mrad/pass) ×v _(c) +lwhere D_(Mrad/min) is the dose rate in Mrad/min, D_(Mrad/pass) is thedose rate in Mrad/pass, v_(c) is the conveyor speed and l is the lengthof the specimen that travels through the c-beam raster area. Whenelectron irradiation is used, the energy of the electrons can be variedto change the depth of penetration of the electrons. Preferably, theenergy of the electrons is about 0.5 MeV to about 12 MeV, morepreferably about 5 MeV to about 12 MeV. Such manipulability isparticularly useful when the irradiated object is an article of varyingthickness or depth, e.g., an articular cup for a medical prosthesis.

The irradiated UHMWPE is heated above the melting temperature of theUHMWPE so that there are no detectable free radicals in the UHMWPE. Theheating provides the molecules with sufficient mobility so as toeliminate the constraints derived from the crystals of the UHMWPE,thereby allowing essentially all of the residual free radicals torecombine. Preferably, the UHMWPE is heated to a temperature of about137° C. to about 300° C., more preferably about 140° C. to about 300°C., more preferably yet about 140° C. to about 190° C., more preferablyyet about 145° C. to about 300° C., more preferably yet about 145° C. toabout 190° C., more preferably yet about 146° C. to about 190° C., andmost preferably about 150° C. Preferably, the temperature in the heatingstep is maintained for about 0.5 minutes to about 24 hours, morepreferably about 1 hour to about 3 hours, and most preferably about 2hours. The heating can be carried out, e.g., in air, in an inert gas,e.g., nitrogen, argon or helium, in a sensitizing atmosphere, e.g.,acetylene, or in a vacuum. It is preferred that for the longer heatingtimes, that the heating be carried out in an inert gas or under vacuum.

Another preferred embodiment of this method is called WIR-SM, i.e., warmirradiation and subsequent melting. In this embodiment, the UHMWPE thatis provided is pre-heated to a temperature below the melting temperatureof the UHMWPE. The pre-heating can be done in an inert or non-inertenvironment. It is preferred that this pre-heating is done in air.Preferably, the UHMWPE is pre-heated to a temperature of about 20° C. toabout 135° C., more preferably to a temperature greater than about 20°C. to about 135° C., and most preferably to a temperature of about 50°C. The other parameters are as described above for the CIR-SMembodiment, except that the dose rate for the irradiating step usingelectron irradiation is preferably about 0.05 to about 10 Mrad/minute,and more preferably is about 4 to about 5 Mrad/minute; and the dose ratefor the irradiating step using gamma irradiation is preferably about0.05 to about 0.2 Mrad/minute, and more preferably is about 0.2Mrad/minute.

Another preferred embodiment of this method is called WIR-AM, i.e., warmirradiation and adiabatic melting. In this embodiment, the UHMWPE thatis provided is pre-heated to a temperature below the melting temperatureof the UHMWPE. The pre-heating can be done in an inert or non-inertenvironment. It is preferred that this pre-heating is done in air. Thepre-heating can be done, e.g., in an oven. It is preferred that thepre-heating is to a temperature between about 100° C. to below themelting temperature of the UHMWPE. Preferably, the UHMWPE is pre-heatedto a temperature of about 100° C. to about 135° C., more preferably thetemperature is about 130° C., and most preferably is about 120° C.Preferably, the UHMWPE is in an insulating material so as to reduce heatloss from the UHMWPE during processing. The heat is meant to include,e.g., the pre-heat delivered before irradiation and the heat generatedduring irradiation. By insulating material is meant any type of materialwhich has insulating properties, e.g., a fiberglass pouch.

The pre-heated UHMWPE is then irradiated to a high enough total dose andat a fast enough dose rate so as to generate enough heat in the polymerto melt substantially all the crystals in the material and thus ensureelimination of substantially all detectable free radicals generated by,e.g., the irradiating step. It is preferred that the irradiating stepuse electron irradiation so as to generate such adiabatic heating. Byadiabatic heating is meant no loss of heat to the surroundings duringirradiation. Adiabatic heating results in adiabatic melting if thetemperature is above the melting point. Adiabatic melting is meant toinclude complete or partial melting. The minimum total dose isdetermined by the amount of heat necessary to heat the polymer from itsinitial temperature (i.e., the pre-heated temperature discussed above)to its melting temperature, and the heat necessary to melt all thecrystals, and the heat necessary to heat the polymer to a pre-determinedtemperature above its melting point. The following equation describeshow the amount of total dose is calculated:Total Dose=c _(p)

(T _(m) −T _(i))+ΔH _(m) +c _(p)

(T _(f) −T _(m))where c_(p)

(=2 J/g/° C.) and c_(p)

(=3 J/g/° C.) are heat capacities of UHMWPE in the solid state and meltstate, respectively, ΔH_(m) (=146 J/g) is the heat of melting of theunirradiated Hoescht Celanese CUR 415 bar stock, T_(i) is the initialtemperature, and T_(f) is the final temperature. The final temperatureshould be above the melting temperature of the UHMWPE.

Preferably, the final temperature of the UHMWPE is about 140° C. toabout 200° C., more preferably it is about 145° C. to about 190° C.,more preferably yet it is about 146° C. to about 190° C., and mostpreferably it is about 150° C. At above 160° C., the polymer starts toform bubbles and cracks. Preferably, the dose rate of the electronirradiation is about 2 to about 3,000 Mrad/minute, more preferably yetis about 2 to about 30 Mrad/minute, more preferably yet is about 7 toabout 25 Mrad/minute, more preferably yet is about 20 Mrad/minute, andmost preferably is about 7 Mrad/minute. Preferably, the total absorbeddose is about 1 to about 100 Mrad. Using the above equation, theabsorbed dose for an initial temperature of 130° C. and a finaltemperature of 150° C. is calculated to be about 22 Mrad.

In this embodiment, the heating step of the method results from theadiabatic heating described above.

In certain embodiments, the adiabatic heating completely melts theUHMWPE. In certain embodiments, the adiabatic heating only partiallymelts the UHMWPE. Preferably, additional heating of the irradiatedUHMWPE is done subsequent to the irradiation induced adiabatic heatingso that the final temperature of the UHMWPE after the additional heatingis above the melting temperature of the UHMWPE, so as to ensure completemelting of the UHMWPE. Preferably, the temperature of the UHMWPE fromthe additional heating is about 140° C. to about 200° C., morepreferably is about 145° C. to about 190° C., more preferably yet isabout 146° C. to about 190° C., and most preferably is about 150° C.

Yet another embodiment of this invention is called CIR-AM, i.e., coldirradiation and adiabatic heating. In this embodiment, UHMWPE at roomtemperature or below room temperature is melted by adiabatic heating,with or without subsequent additional heating, as described above.

This invention also includes the product made in accordance with theabove described method.

Also provided in this invention is a method of making a medicalprosthesis from UHMWPE having substantially no detectable free radicals,the prosthesis resulting in the reduced production of particles from theprosthesis during wear of the prosthesis. Radiation treated UHMWPEhaving no detectable free radicals is provided. A medical prosthesis isformed from this UHMWPE so as to reduce production of particles from theprosthesis during wear of the prosthesis, the UHMWPE forming a loadbearing surface of the prosthesis. Formation of the prosthesis can beaccomplished by standard procedures known to those skilled in the art,e.g., machining.

Also provided in this invention is a method of treating a body in needof a medical prosthesis. A shaped prosthesis formed of radiation treatedUHMWPE having substantially no detectable free radicals is provided.This prosthesis is applied to the body in need of the prosthesis. Theprosthesis reduces production of fine particles from the prosthesisduring wear of the prosthesis. In preferred embodiments, the ultra highmolecular weight polyethylene forms a load bearing surface of theprosthesis.

In yet another embodiment of this invention, a medical prosthesis foruse within the body which is formed of ultra high molecular weightpolyethylene (UHMWPE) which has a polymeric structure with less thanabout 50% crystallinity, less than about 290 Å lamellar thickness andless than about 940 MPa tensile elastic modulus, so as to reduceproduction of fine particles from the prosthesis during wear of theprosthesis, is provided.

The UHMWPE of this embodiment has a polymeric structure with less thanabout 50% crystallinity, preferably less than about 40% crystallinity.By crystallinity is meant the fraction of the polymer that iscrystalline. The crystallinity is calculated by knowing the weight ofthe sample (w, in g), the heat absorbed by the sample in melting (E, incal) and the calculated heat of melting of polyethylene in the 100%crystalline state (ΔH°=69.2 cal/g), and using the following equation:${\%\quad{crystallinity}} = \frac{E}{{w \cdot \Delta}\quad{H{^\circ}}}$

The UHMWPE of this embodiment has a polymeric structure with less thanabout 290 Å lamellar thickness, preferably less than about 200 Ålamellar thickness, and most preferably less than about 100 Å lamellarthickness. By lamellar thickness (l) is meant the calculated thicknessof assumed lamellar structures in the polymer using the followingexpression:$1 = \frac{{2 \cdot \sigma_{e} \cdot T_{m}}{^\circ}}{\Delta\quad{{H{^\circ}} \cdot ( {{T_{m}{^\circ}} - T_{m}} ) \cdot \rho}}$where, σ_(e) is the end free surface energy of polyethylene (2.22×10⁻⁶cal/cm²), ΔH° is the calculated heat of melting of polyethylene in the100% crystalline state (69.2 cal/g), p is the density of the crystallineregions (1.005 g/cm³), T_(m)° is the melting point of a perfectpolyethylene crystal (418.15K) and T_(m) is the experimentallydetermined melting point of the sample.

The UHMWPE of this embodiment has less than about 940 MPa tensileelastic modulus, preferably less than about 600 MPa tensile elasticmodulus, more preferably less than about 400 MPa tensile elasticmodulus, and most preferably less than about 200 MPa tensile elasticmodulus. By tensile elastic modulus is meant the ratio of the nominalstress to corresponding strain for strains less than 0.5% as determinedusing the standard test ASTM 638 M III.

Preferably, the UHMWPE of this embodiment has a polymeric structure withabout 40% crystallinity, about 100 Å lamellar thickness and about 200MPa tensile elastic modulus.

The UHMWPE of this embodiment has no trapped free radicals, e.g.,unsaturated trans-vinylene free radicals. It is preferred that theUHMWPE of this embodiment have a hardness less than about 65 on theShore D scale, more preferably a hardness less than about 55 on theShore D scale, most preferably a hardness less than about 50 on theShore D scale. By hardness is meant the instantaneous indentationhardness measured on the Shore D scale using a durometer described inASTM D2240. It is preferred that the UHMWPE of this embodiment besubstantially not oxidized. The polymeric structure has extensivecross-linking such that a substantial portion of the polymeric structuredoes not dissolve in Decalin. By substantial portion is meant at least50% of the polymer sample's dry weight. By not dissolve in Decalin ismeant does not dissolve in Decalin at 150° C. over a period of 24 hours.Preferably, The UHMWPE of this embodiment has a high density ofentanglement so as to cause the formation of imperfect crystals andreduce crystallinity. By the density of entanglement is meant the numberof points of entanglement of polymer chains in a unit volume; a higherdensity of entanglement being indicated by the polymer sample'sinability to crystallize to the same extent as conventional UHMWPE, thusleading to a lesser degree of crystallinity.

The invention also includes other fabricated articles made from theUHMWPE of this embodiment having a polymeric structure with less thanabout 50% crystallinity, less than about 290 Å lamellar thickness andless than about 940 MPa tensile elastic modulus. Such articles includeshaped articles and unshaped articles, including, e.g., machinedobjects, e.g., cups, gears, nuts, sled runners, bolts, fasteners,cables, pipes and the like, and bar stock, films, cylindrical bars,sheeting, panels, and fibers Shaped articles can be made, e.g., bymachining. The fabricated articles are particularly suitable for loadbearing applications, e.g., as a load bearing surface, and as metalreplacement articles. Thin films or sheets of UHMWPE, which have beenmelt-irradiated can also be attached, e.g., with glue, onto supportingsurfaces, and thus used as a transparent, wear resistant load bearingsurface.

The invention also includes an embodiment in which UHMWPE has a uniquepolymeric structure characterized by less than about 50% crystallinity,less than about 290 Å lamellar thickness and less than about 940 MPatensile elastic modulus. Depending upon the particular processing usedto make the UHMWPE, certain impurities may be present in the UHMWPE ofthis invention, including, e.g., calcium stearate, mold release agents,extenders, anti-oxidants and/or other conventional additives topolyethylene polymers. In certain embodiments, the UHMWPE has hightransmissivity of light, preferably a transmission greater than about10% of light at 517 nm through a 1 mm thick sample, more preferably atransmission greater than about 30% of light at 517 nm through a 1 mmthick sample, and most preferably a transmission greater than about 40%of light at 517 nm through a 1 mm thick sample. Such UHMWPE isparticularly useful for thin films or sheets which can be attached ontosupporting surfaces of various articles, the film or sheet beingtransparent and wear resistant.

In another embodiment of this invention, a method for making crosslinkedUHMWPE is provided. This method is called melt irradiation (MIR).Conventional UHMWPE is provided. Preferably, the UHMWPE is surroundedwith an inert material that is substantially free of oxygen. The UHMWPEis heated above the melting temperature of the UHMWPE so as tocompletely melt all crystalline structure. The heated UHMWPE isirradiated, and the irradiated UHMWPE is cooled to about 25° C.

Preferably, the UHMWPE made from this embodiment has a polymericstructure with less than about 50% crystallinity, less than about 290 Ålamellar thickness and less than about 940 MPa tensile elastic modulus.Conventional UHMWPE, e.g., a bar stock, a shaped bar stock, a coating,or a fabricated article is provided. By conventional UHMWPE is meantcommercially available high density (linear) polyethylene of molecularweights greater than about 500,000. Preferably, the UHMWPE startingmaterial has an average molecular weight of greater than about 2million. By initial average molecular weight is meant the averagemolecular weight of the UHMWPE starting material, prior to anyirradiation. It is preferred that this UHMWPE is surrounded with aninert material that is substantially free of oxygen, e.g., nitrogen,argon or helium. In certain embodiments, a non-inert environment can beused. The UHMWPE is heated above its melting temperature for a timesufficient to allow all the crystals to melt. Preferably, thetemperature is about 145° C. to about 230° C., and more preferably, isabout 175° to about 200° C. Preferably, the heating is maintained so tokeep the polymer at the preferred temperature for about 5 minutes toabout 3 hours, and more preferably for about 30 minutes to about 2hours. The UHMWPE is then irradiated with gamma irradiation or electronirradiation. In general, gamma irradiation gives a high penetrationdepth but takes a longer time, resulting in the possibility of someoxidation. In general, electron irradiation gives more limitedpenetration depths but takes a shorter time, and hence the possibilityof oxidation is reduced. The irradiation dose can be varied to controlthe degree of crosslinking and crystallinity in the final UHMWPEproduct. Preferably, a dose of greater than about 1 Mrad is used, morepreferably a dose of greater than about 20 Mrad is used. When electronirradiation is used, the energy of the electrons can be varied to changethe depth of penetration of the electrons, thereby controlling thedegree of crosslinking and crystallinity in the final UHMWPE product.Preferably, the energy is about 0.5 MeV to about 12 MeV, more preferablyabout 1 MeV to about 10 MeV, and most preferably about 10 MeV. Suchmanipulability is particularly useful when the irradiated object is anarticle of varying thickness or depth, e.g., an articular cup for aprosthesis. The irradiated UHMWPE is then cooled to about 25° C.Preferably, the cooling rate is equal to or greater than about 0.5°C./min, more preferably equal to or greater than about 20° C./min. Incertain embodiments, the cooled UHMWPE can be machined. In preferredembodiments, the cooled irradiated UHMWPE has substantially nodetectable free radicals. Examples 1, 3 and 6 describe certain preferredembodiments of the method. Examples 2, 4 and 5, and FIGS. 4 through 7,illustrate certain properties of the melt-irradiated UHMWPE obtainedfrom these preferred embodiments, as compared to conventional UHMWPE.

This invention also includes the product made in accordance with theabove described method.

In an embodiment of MIR, highly entangled and crosslinked UHMWPE ismade. Conventional UHMWPE is provided. Preferably, the UHMWPE issurrounded with an inert material that is substantially free of oxygen.The UHMWPE is heated above the melting temperature of the UHMWPE for atime sufficient to enable the formation of entangled polymer chains inthe UHMWPE. The heated UHMWPE is irradiated so as to trap the polymerchains in the entangled state. The irradiated UHMWPE is cooled to about25° C.

This invention also includes the product made in accordance with theabove described method.

Also provided in this invention is a method of making a prosthesis fromUHMWPE so as to reduce production of fine particles from the prosthesisduring wear of the prosthesis. UHMWPE having a polymeric structure withless than about 50% crystallinity, less than about 290 Å lamellarthickness and less than about 940 MPa tensile elastic modulus isprovided. A prosthesis is formed from this UHMWPE, the UHMWPE forming aload bearing surface of the prosthesis. Formation of the prosthesis canbe accomplished by standard procedures known to those skilled in theart, e.g., machining.

Also provided in this invention is a method of treating a body in needof a prosthesis. A shaped prosthesis formed of ultra high molecularweight polyethylene having a polymeric structure with less than about50% crystallinity, less than about 290 Å lamellar thickness and lessthan about 940 MPa tensile elastic modulus, is provided. This prosthesisis applied to the body in need of the prosthesis. The prosthesis reducesproduction of fine particles from the prosthesis during wear of theprosthesis. In preferred embodiments, the ultra high molecular weightpolyethylene forms a load bearing surface of the prosthesis.

The products and processes of this invention also apply to otherpolymeric materials such as high-density-polyethylene,low-density-polyethylene, linear-low-density-polyethylene andpolypropylene.

The following non-limiting examples further illustrate the presentinvention.

EXAMPLES Example 1 Method of Making Melt-Irradiated UHMWPE (MIR)

This example illustrates electron irradiation of melted UHMWPE.

A cuboidal specimen (puck) of size 10 mm×12 mm×60 mm, prepared fromconventional ram extruded UHMWPE bar stock (Hoescht Celanese GUR 415 barstock obtained from Westlake Plastics, Lenni, Pa.) was placed in achamber. The atmosphere within the chamber consisted of low oxygennitrogen gas (<0.5 μm oxygen gas) (obtained from AIRCO, Murray Hill,N.J.). The pressure in the chamber was approximately 1 atm. Thetemperature of the sample and the irradiation chamber was controlledusing a heater, a variac and a thermocouple readout (manual) ortemperature controller (automatic). The chamber was heated with a 270 Wheating mantle. The chamber was heated (controlled by the variac) at arate such that the steady state temperature of the sample was about 175°C. The sample was held at the steady state temperature for 30 minutesbefore starting the irradiation.

Irradiation was done using a van de Graaff generator with electrons ofenergy 2.5 MeV and a dose rate of 1.67 MRad/min. The sample was given adose of 20 MRad with the electron beam hitting the sample on the 60mm×12 mm surface. The heater was switched off after irradiation, and thesample was allowed to cool within the chamber under inert atmosphere,nitrogen gas, to 25° C. at approximately 0.5° C./minute. As a control,similar specimens were prepared using unheated and unirradiated barstock of conventional UHMWPE.

Example 2 Comparison of Properties of GUR 415 UHMWPE Bar Stock andMelt-Irradiated (MIR) GUR 415 UHMWPE Bar Stock (20 MRad)

This example illustrates various properties of the irradiated andunirradiated samples of UHMWPE bar stock (GUR 415) obtained fromExample 1. The tested samples were as follows: the test sample was barstock which was molten and then irradiated while molten; control was barstock (no heating/melting, no irradiation).

(A) Differential Scanning Calorimetry (DSC)

A Perkin-Elmer DSC7 was used with an ice-water heat sink and a heatingand cooling rate of 10° C./minute with a continuous nitrogen purge. Thecrystallinity of the samples obtained from Example 1 was calculated fromthe weight of the sample and the heat of melting of polyethylenecrystals (69.2 cal/g). The temperature corresponding to the peak of theendotherm was taken as the melting point. The lamellar thickness wascalculated by assuming a lamellar crystalline morphology, and knowingΔH° the heat of melting of 100% crystalline polyethylene (69.2 cal/g),the melting point of a perfect crystal (418.15 K), the density of thecrystalline regions (1.005 g/cm³) and the end free surface energy ofpolyethylene (2.22×10⁻⁶ cal/cm²). The results are shown in Table 1 andFIG. 4. TABLE 1 DSC (10° C./min) GUR 415 GUR 415 (unirradiated)(melt-irradiated) Property 0 MRad 20 MRad Crystallinity (%) 50.2 37.8Melting Point (C.) 135.8 125.5 Lamellar thickness (Å) 290 137

The results indicate that the melt-irradiated sample had a moreentangled and less crystalline polymeric structure than the unirradiatedsample, as evidenced by lower crystallinity, lower lamellar thicknessand lower melting point.

(B) Swell Ratio

The samples were cut into cubes of size 2 mm×2 mm×2 mm and keptsubmerged in Decalin at 150° C. for a period of 24 hours. An antioxidant(1% N-phenyl-2-naphthylamine) was added to the Decalin to preventdegradation of the sample. The swell ratio and percent extract werecalculated by measuring the weight of the sample before the experiment,after swelling for 24 hours and after vacuum drying the swollen sample.The results are shown in Table 2. TABLE 2 Swelling in Decalin withAntioxidant for 24 hours at 150° C. GUR 415 GUR 415 (unirradiated)(melt-irradiated) Property 0 MRad 20 MRad Swell Ratio dissolves 2.5Extract (%) approx. 100% 0.0

The results indicate that the melt-irradiated UHMWPE sample was highlycrosslinked, and hence did not allow dissolution of polymer chains intothe hot solvent even after 24 hours, while the unirradiated sampledissolved completely in the hot solvent in the same period.

(C) Tensile Elastic Modulus

ASTM 638 M III of the samples was followed. The displacement rate was 1mm/minute. The experiment was performed on a MTS machine. The resultsare shown in Table 3. TABLE 3 Elastic Test (ASTM 638 M III, 1 mm/min.GUR 415 GUR 415 (unirradiated) (melt-irradiated) Property 0 MRad 20 MRadTensile Elastic modulus (MPa) 940.7 200.8 Yield stress 22.7 14.4 Strainat break (%) 953.8 547.2 Engineering UTS (MPa) 46.4 15.4

The results indicate that the melt-irradiated UHMWPE sample had asignificantly lower tensile elastic modulus than the unirradiatedcontrol The lower strain at break of the melt-irradiated UHMWPE sampleis yet further evidence for the crosslinking of chains in that sample.

(D) Hardness

The hardness of the samples was measured using a durometer on the shoreD scale. The hardness was recorded for instantaneous indentation. Theresults are shown in Table 4. TABLE 4 Hardness (Shore D) GUR 415 GUR 415(unirradiated) (melt-irradiated) Property 0 MRad 20 MRad Hardness (DScale) 65.5 54.5

The results indicate that the melt-irradiated UHMWPE was softer than theunirradiated control.

(E) Light Transmissivity (Transparency)

Transparency of the samples was measured as follows: Light transmissionwas studied for a light of wave length 517 nm passing through a sampleof approximately 1 mm in thickness placed between two glass slides. Thesamples were prepared by polishing the surfaces against 600 grit paper.Silicone oil was spread on the surfaces of the sample and then thesample was placed in between two slides. The silicone oil was used inorder to reduce diffuse light scattering due to the surface roughness ofthe polymer sample. The reference used for this purpose was two similarglass slides separated by a thin film of silicone oil. Thetransmissivity was measured using a Perkin Elmer Lambda 3B uv-visspectrophotometer. The absorption coefficient and transmissivity of asample exactly 1 mm thick were calculated using the Lambert-Beer law.The results are shown in Table 5. TABLE 5 Transmissivity of Light at 517nm GUR 415 GUR 415 (unirradiated) (melt-irradiated) Property 0 MRad 20MRad Transmission (%) 8.59 39.9 (1 mm sample) Absorption coefficient24.54 9.18 (cm¹)

The results indicate that the melt-irradiated UHMWPE sample transmittedmuch more light through it than the control, and hence is much moretransparent than the control.

(F) Environmental Scanning Electron Microscopy (ESEM)

ESEM (ElectroScan, Model 3) was performed on the samples at 10 kV (lowvoltage to reduce radiation damage to the sample) with an extremely thingold coating (approximately 20 Å to enhance picture quality). Bystudying the surface of the polymer under the ESEM with and without thegold coating, it was verified that the thin gold coating did not produceany artifacts.

The samples were etched using a permanganate etch with a 1:1 sulfuricacid to orthophosphoric acid ratio and a 0.7% (w/v) concentration ofpotassium permanganate before being viewed under the ESEM.

FIG. 4 shows an ESEM (magnification of 10,000×) of an etched surface ofconventional UHMWPE (GUR 415; unheated; unirradiated). FIG. 5 shows anESEM (magnification of 10,500×) of an etched surface of melt-irradiatedUHMWPE (GUR 415; melted; 20 MRad). The ESEMs indicated a reduction insize of the crystallites and the occurrence of imperfect crystallizationin the melt-irradiated UHMWPE as compared to the conventional UHMWPE.

(G) Fourier Transform Infra Red Spectroscopy (FTIR)

FTIR of the samples was performed using a microsampler on the samplesrinsed with hexane to remove surface impurities. The peaks observedaround 1740 to 1700 cm⁻¹ are bands associated with oxygen containinggroups. Hence, the ratio of the area under the carbonyl peak at 1740cm⁻¹ to the area under the methylene peak at 1460 cm⁻¹ is a measure ofthe degree of oxidation.

The FTIR spectra indicate that the melt-irradiated UHMWPE sample showedmore oxidation than the conventional unirradiated UHMWPE control, but alot less oxidation than an UHMWPE sample irradiated in air at roomtemperature and given the same irradiation dose as the melt-irradiatedsample.

(H) Electron Paramagnetic Resonance (EPR)

EPR was performed at room temperature on the samples which were placedin a nitrogen atmosphere in an air tight quartz tube. The instrumentused was the Bruker ESP 300 EPR spectrometer and the tubes used wereTaperlok EPR sample tubes obtained from Wilmad Glass Company, Buena,N.J.

The unirradiated samples do not have any free radicals in them sinceirradiation is the process which creates free radicals in the polymer.On irradiation, free radicals are created which can last for severalyears under the appropriate conditions.

The EPR results indicate that the melt-irradiated sample did not showany free radicals when studied using an EPR immediately afterirradiation, whereas the sample which was irradiated at room temperatureunder nitrogen atmosphere showed trans-vinylene free radicals even after266 days of storage at room temperature. The absence of free radicals inthe melt-irradiated UHMWPE sample means that any further oxidativedegradation was not possible.

(I) Wear

The wear resistance of the samples was measured using a bi-axialpin-on-disk wear tester. The wear test involved the rubbing action ofUHMWPE pins (diameter=9 mm; height=13 mm) against a Co—Cr alloy disk.These tests were carried out to a total of 2 million cycles. Theunirradiated pin displayed a wear rate of 8 mg/million-cycles while theirradiated pin had a wear rate of 0.5 mg/million cycles. The resultsindicate that the melt-irradiated UHMWPE has far superior wearresistance than the unirradiated control.

Example 3 Method of Making Melt-Irradiated (MIR) UHMWPE ConventionalArticular Cups

This example illustrates electron irradiation of a melted UHMWPEconventional articular cup.

A conventional articular cup (high conformity unsterilized UHMWPE cupmade by Zimmer, Inc., Warsaw, Ind.) of internal diameter 26 mm and madeof GUR 415 ram extruded bar stock, was irradiated under controlledatmosphere and temperature conditions in an air-tight chamber with atitanium cup holder at the base and a thin stainless steel foil (0.001inches thick) at the top. The atmosphere within this chamber consistedof low oxygen nitrogen gas (<0.5 ppm oxygen gas) (obtained from AIRCO,Murray Hill, N.H.). The pressure in the chamber was approximately 1 atm.The chamber was heated using a 270 W heating mantle at the base of thechamber which was controlled using a temperature controller and avariac. The chamber was heated such that the temperature at the topsurface of the cup rose at approximately 1.5° to 2° C./min, finallyasymptotically reaching a steady state temperature of approximately 175°C. Due to the thickness of the sample cup and the particular design ofthe equipment used, the steady state temperature of the cup variedbetween 200° C. at the base to 175° C. at the top. The cup was held atthese temperatures for a period of 30 minutes before starting theirradiation.

Irradiation was done using a van de Graaff generator with electrons ofenergy 2.5 MeV and a dose rate of 1.67 MRad/min. The beam entered thechamber through the thin foil at top and hit the concave surface of thecup. The dose received by the cup was such that a maximum dose of 20MRad was received approximately 5 mm below the surface of the cup beinghit by the electrons. After irradiation, the heating was stopped and thecup was allowed to cool to room temperature (approximately 25° C.) whilestill in the chamber with nitrogen gas. The rate of cooling wasapproximately 0.5° C./min. The sample was removed from the chamber afterthe chamber and the sample had reached room temperature.

The above irradiated cup which increases in volume (due to the decreasein density accompanying the reduction of crystallinity followingmelt-irradiation) can be remachined to the appropriate dimensions.

Example 4 Swell Ratio and Percent Extract at Different Depths forMelt-Irradiated (MIR) UHMWPE Articular Cups

This example illustrates the swell ratio and percent extract atdifferent depths of the melt-irradiated articular cup obtained fromExample 3. Samples of size 2 mm×2 mm×2 mm were cut from the cup atvarious depths along the axis of the cup. These samples were then keptsubmerged in Decalin at 150° C. for a period of 24 hours. An antioxidant(1% N-phenyl-2-naphthylamine) was added to the Decalin to preventdegradation of the sample. The swell ratio and percent extract werecalculated by measuring the weight of the sample before the experiment,after swelling for 24 hours, and after vacuum drying the swollen sample.The results are shown in Table 6. TABLE 6 The Swell Ratio and PercentExtract at Different Depths on the Melt-Irradiated UHMWPE Articular CupSwell Ratio Depth (mm) (Decalin, 150° C., 1 day) % Extract 0-2 2.43 0.02-4 2.52 0.0 4-6 2.51 0.0 6-8 2.64 0.0  8-10 2.49 0.0 10-12 3.68 0.0 >126.19 35.8  Unirradiated Dissolves Approx. 100%

The results indicate that the UHMWPE in the cup had been crosslinked toa depth of 12 mm due to the melt-irradiation process to such an extentthat no polymer chains dissolved out in hot Decalin over 24 hours.

Example 5 Crystallinity and Melting Point at Different Depths for theMelt-Irradiated (MIR) UHMWPE Articular Cups

This example illustrates the crystallinity and melting point atdifferent depths of the melt-irradiated cup obtained from Example 3.

Samples were taken from the cup at various depths along the axis of thecup. The crystallinity is the fraction of the polymer that iscrystalline. The crystallinity was calculated by knowing the weight ofthe sample (w, in g), the heat absorbed by the sample in melting (E, incal which was measured experimentally using a Differential ScanningCalorimeter at 10° C./min) and the heat of melting of polyethylene inthe 100% crystalline state (ΔH°=69.2 cal/g), using the followingequation:${\%\quad{crystallinity}} = \frac{E}{{w \cdot \Delta}\quad{H{^\circ}}}$

The melting point is the temperature corresponding to the peak in theDSC endotherm. The results are shown in FIG. 7.

The results indicate that the crystallinity and the melting point of themelt-irradiated UHMWPE in the articular cups obtained from Example 3were much lower than the corresponding values of the conventionalUHMWPE, even to a depth of 1 cm (the thickness of the cup being 1.2cms).

Example 6 Second Method of Making Melt-Irradiated (MIR) UHMWPE ArticularCups

This example illustrates a method for making articular cups withmelt-irradiated UHMWPE.

Conventional ram extruded UHMWPE bar stock (GUR 415 bar stock obtainedfrom West Lake Plastics, Lenni, Pa.) was machined to the shape of acylinder, of height 4 cm and diameter 5.2 cm. One circular face of thecylinder was machined to include an exact hemispherical hole, ofdiameter 2.6 cm, such that the axis of the hole and the cylindercoincided. This specimen was enclosed in an air-tight chamber with athin stainless steel foil (0.001 inches thick) at the top. Thecylindrical specimen was placed such that the hemispherical hole facedthe foil. The chamber was then flushed and filled with an atmosphere oflow oxygen nitrogen gas (<0.5 ppm oxygen gas) obtained from AIRCO,Murray Hill, N.J.). Following this flushing and filling, a slowcontinuous flow of nitrogen was maintained while keeping the pressure inthe chamber at approximately 1 atm. The chamber was heated using a 270 Wheating mantle at the base of the chamber which was controlled using atemperature controller and a variac. The chamber was heated such thatthe temperature at the top surface of the cylindrical specimen rose atapproximately 1.5° C. to 2° C./min, finally asymptotically reaching asteady state temperature of approximately 175° C. The specimen was thenheld at this temperature for a period of 30 minutes before startingirradiation.

Irradiation was done using a van de Graaff generator with electrons ofenergy 2.5 MeV and a dose rate of 1.67 MRad/min. The beam entered thechamber through the thin foil at top and hit the surface with thehemispherical hole. The dose received by the specimen was such that amaximum dose of 20 MRad was received approximately 5 mm below thesurface of the polymer being hit by the electrons. After irradiation,the heating was stopped and the specimen was allowed to cool to roomtemperature (approximately 25° C.) while still in the chamber withnitrogen gas. The rate of cooling was approximately 0.5° C./min. Thesample was removed from the chamber after the chamber and the sample hadreached room temperature.

This cylindrical specimen was then machined into an articular cup withthe dimensions of a high conformity UHMWPE articular cup of internaldiameter 26 mm manufactured by Zimmer, Inc., Warsaw, Ind., such that theconcave surface of the hemispherical hole was remachined into thearticulating surface. This method allows for the possibility ofrelatively large changes in dimensions during melt irradiation.

Example 7 Electron Irradiation of UHMWPE Pucks

This example illustrates that electron irradiation of UHMWPE pucks givesa non-uniform absorbed dose profile.

Conventional UHMWPE ram extruded bar stock (Hoescht Celanese GUR 415 barstock obtained from Westlake Plastics, Lenni, Pa.) was used. The GUR 415resin used for the bar stock had a molecular weight of 5,000,000 g/moland contained 500 ppm of calcium stearate. The bar stock was cut into“hockey puck” shaped cylinders (height 4 cm, diameter 8.5 cm).

The pucks were irradiated at room temperature with an electron-beamincident to one of the circular bases of the pucks with a linearelectron accelerator operated at 10 MeV and 1 kW (AECL, Pinawa,Manitoba, Canada), with a scan width of 30 cm and a conveyor speed of0.08 cm/sec. Due to a cascade effect, electron beam irradiation resultsin a non-uniform absorbed dose profile. Table 7 illustrates thecalculated absorbed dose values at various depths in a specimen ofpolyethylene irradiated with 10 MeV electrons. The absorbed doses werethe values measured at the top surface (surface of e-beam incidence).TABLE 7 The variation of absorbed dose as a function of depth inpolyethylene Depth (mm) Absorbed Dose (Mrad) 0 20 0.5 22 1.0 23 1.5 242.0 25 2.5 27 3.0 26 3.5 23 4.0 20 4.5 8 5.0 3 5.5 1 6.0 0

Example 8 Method of Making UHMWPE Using Cold Irradiation and SubsequentMelting (CIR-SM)

This example illustrates a method of making UHMWPE that has across-linked structure and has substantially no detectable freeradicals, by cold irradiating and then melting the UHMWPE.

Conventional UHMWPE ram extruded bar stock (Hoescht Celanese GUR 415 barstock obtained from Westlake Plastics, Lenni, Pa.) was used. The GUR 415resin used for the bar stock had a molecular weight of 5,000,000 g/moland contained 500 ppm of calcium stearate. The bar stock was cut into“hockey puck” shaped cylinders (height 4 cm, diameter 8.5 cm).

The pucks were irradiated at room temperature at a dose rate of 2.5 Mradper pass to 2.5, 5, 7.5, 10, 12.5, 15, 17.5, 20, 30, and 50 Mrad totalabsorbed dose as measured on the top surface (electron-beam incidence)(AECL, Pinawa, Manitoba, Canada). The pucks were not packaged and theirradiation was carried out in air. Subsequent to irradiation, the puckswere heated to 150° C. under vacuum for 2 hours so as to melt thepolymer and thereby result in the recombination of free radicals leadingto substantially no detectable residual free radicals. The pucks werethen cooled to room temperature at a rate of 5° C./min.

The residual free radicals are measured by electron paramagneticresonance as described in Jahan et al., J. Biomedical Materials Research25: 1005 (1991).

Example 9 Method of Making UHMWPE Using Warm Irradiation and SubsequentMelting (WIR-SM)

This example illustrates a method of making UHMWPE that has across-linked structure and has substantially no detectable freeradicals, by irradiating UHMWPE that has been heated to below themelting point, and then melting the UHMWPE.

Conventional UHMWPE ram extruded bar stock (Hoescht Celanese GUR 415 barstock obtained from Westlake Plastics, Lenni, Pa.) was used. The GUR 415resin used for the bar stock had a molecular weight of 5,000,000 g/moland contained 500 ppm of calcium stearate. The bar stock was cut into“hockey puck” shaped cylinders (height 4 cm, diameter 8.5 cm).

The pucks were heated to 100° C. in air in an oven. The heated puckswere then irradiated with an electron beam to a total dose of 20 Mrad ata dose rate of 2.5 Mrad per pass (E-Beam Services, Cranbury, N.J.), witha scan width of 30 cm and a conveyor speed of 0.08 cm/sec. Subsequent toirradiation, the pucks were heated to 150° C. under vacuum for 2 hours,thereby allowing the free radicals to recombine leading to substantiallyno detectable residual free radicals. The pucks were then cooled to roomtemperature at a rate of 5° C./min.

Example 10 Method of Making UHMWPE Using Warm Irradiation and AdiabaticMelting (WIR-AM)

This example illustrates a method of making UHMWPE that has across-linked structure and has substantially no detectable freeradicals, by irradiating UHMWPE that has been heated to below themelting point so as to generate adiabatic melting of the UHMWPE.

Conventional UHMWPE ram extruded bar stock (Hoescht Celanese GUR 415 barstock obtained from Westlake Plastics, Lenni, Pa.) was used. The GUR 415resin used for the bar stock had a molecular weight of 5,000,000 g/moland contained 500 ppm of calcium stearate. The bar stock was cut into“hockey puck” shaped cylinders (height 4 cm, diameter 8.5 cm).

Two pucks were packed in a fiberglass pouch (obtained from FisherScientific Co., Pittsburgh, Pa.) to minimize heat loss in subsequentprocessing steps. First, the wrapped pucks were heated overnight in anair convection oven kept at 120° C. As soon as the pucks were removedfrom the oven they were placed under an electron-beam incident to one ofthe circular bases of the pucks from a linear electron acceleratoroperated at 10 MeV and 1 kW (AECL, Pinawa, Manitoba, Canada), andimmediately irradiated to a total dose of 21 and 22.5 Mrad,respectively. The dose rate was 2.7 Mrad/min. Therefore, for 21 Mrad,radiation was for 7.8 min., and for 22.5 Mrad, radiation was for 8.3min. Following the irradiation, the pucks were cooled to roomtemperature at a rate of 5° C./minute, at which point the fiberglasspouch was removed and the specimens analyzed.

Example 11 Comparison of Properties of GUR 415 UHMWPE Bar Stock Pucksand CIR-SM and WIR-AM-Treated Bar Stock Pucks

This example illustrates various properties of the irradiated andunirradiated samples of UHMWPE bar stock GUR 415 obtained from Examples8 and 10. The tested samples were as follows: (i) test samples (pucks)from bar stock which was irradiated at room temperature, subsequentlyheated to about 150° C. for complete melting of polyethylene crystals,followed by cooling to room temperature (CIR-SM), (ii) test samples(pucks) from bar stock which was heated to 120° C. in a fiberglass pouchso as to minimize heat loss from the pucks, followed by immediateirradiation to generate adiabatic melting of the polyethylene crystals(WIR-AM), and (iii) control bar stock (no heating/melting, noirradiation).

A. Fourier Transform Infra-Red Spectroscopy (FTIR)

Infra-red (IR) spectroscopy of the samples was performed using a BioRadUMA 500 infrared microscope on thin sections of the samples obtainedfrom Examples 8 and 10. The thin sections (50 μm) were prepared with asledge microtome. The IR spectra were collected at 20 μm, 100 μm, and 3mm below the irradiated surface of the pucks with an aperture size of10×50 μm². The peaks observed around 1740 to 1700 cm⁻¹ are associatedwith the oxygen containing groups. Hence, the ratio of the area underthe carbonyl peak at 1740 cm⁻¹ to the area under the methylene peak at1460 cm⁻¹, after subtracting the corresponding baselines, was a measureof the degree of oxidation. Tables 8 and 9 summarize the degree ofoxidation for the specimens described in Examples 8 and 10.

These data show that following the cross-linking procedures there wassome oxidation within a thin layer of about 100 μm thickness. Uponmachining this layer away, the final product would have the sameoxidation levels as the unirradiated control. TABLE 8 Degree ofoxidation of specimens from Example 8 (CIR-SM) (with post-irradiationmelting in vacuum) Oxidation Degree at various depths (A.U.) Specimen 20μm 100 μm 3 mm Unirradiated Control 0.01 0.01 0.02 Irradiated to 2.5Mrad 0.04 0.03 0.03 Irradiated to 5 Mrad 0.04 0.03 0.01 Irradiated to7.5 Mrad 0.05 0.02 0.02 Irradiated to 10 Mrad 0.02 0.03 0.01 Irradiatedto 12.5 Mrad 0.04 0.03 0.01 Irradiated to 15 Mrad 0.03 0.01 0.02Irradiated to 17.5 Mrad 0.07 0.05 0.02 Irradiated to 20 Mrad 0.03 0.020.01

TABLE 9 Degree of oxidation of specimens from Example 10 (WIR-AM)Oxidation Degree at (A.U.) Specimen 20 μm 100 μm 3 mm UnirradiatedControl 0.01 0.01 0.02 Irradiated to 21 Mrad 0.02 0.01 0.03 Irradiatedto 22.5 Mrad 0.02 0.02 0.01

B. Differential Scanning Calorimetry (DSC)

A Perkin-Elmer DSC7 was used with an ice-water heat sink and a heatingand cooling rate of 10° C./minute with a continuous nitrogen purge. Thecrystallinity of the specimens obtained from Examples 8 and 10 wascalculated from the weight of the sample and the heat of melting ofpolyethylene crystals measured during the first heating cycle. Thepercent crystallinity is given by the following equation:${\%\quad{crystallinity}} = \frac{E}{{w \cdot \Delta}\quad{H{^\circ}}}$

where E and w are the heat of melting (J or cal) and weight (grams) ofthe specimen tested, respectively, and ΔH° is the heat of melting of100% crystalline polyethylene in Joules/gram (291 J/g or 69.2 cal/g).The temperature corresponding to the peak of the endotherm was taken asthe melting point. In some cases where there were multiple endothermpeaks, multiple melting points corresponding to these endotherm peakshave been reported. The crystallinities and melting points for thespecimens described in Examples 8 and 10 are reported in Tables 10 and11. TABLE 10 DSC at a heating rate of 10° C./min for specimens ofExample 8 (CIR-SM) Specimen Crystallinity(%) Melting Point(° C.)Unirradiated Control 59 137 Irradiated to 2.5 Mrad 54 137 Irradiated to5 Mrad 53 137 Irradiated to 10 Mrad 54 137 Irradiated to 20 Mrad 51 137Irradiated to 30 Mrad 37 137

TABLE 11 DSC at a heating rate of 10° C./min for specimens of Example 10(WIR-AM) Specimen Crystallinity(%) Melting Point(° C.) UnirradiatedControl 59 137 Irradiated to 21 Mrad 54 120-135-145 Irradiated to 22.5Mrad 48 120-135-145

The data shows that the crystallinity does not change significantly upto absorbed doses of 20 Mrad. Therefore, the elastic properties of thecross-linked material should remain substantially unchanged uponcross-linking. On the other hand, one could tailor the elasticproperties by changing the crystallinity with higher doses. The dataalso shows that the WIR-AM material exhibited three melting peaks.

C. Pin-on-Disc Experiments for Wear Rate

The pin-on-disc (POD) experiments were carried out on a bi-axialpin-on-disc tester at a frequency of 2 Hz where polymeric pins weretested by a rubbing action of the pin against a highly polished Co—Crdisc. Prior to preparing cylindrical shaped pins (height 13 mm, diameter9 mm), one millimeter from the surface of the pucks was machined away inorder to remove the outer layer that had been oxidized duringirradiation and post- and pre-processing. The pins were then machinedfrom the core of the pucks and tested on the POD such that the surfaceof e-beam incidence was facing the Co—Cr disc. The wear tests werecarried out to a total of 2,000,000 cycles in bovine serum. The pinswere weighed at every 500,000 cycle and the average values of weightloss (wear rate) are reported in Tables 12 and 13 for specimens obtainedfrom Examples 0 and 10 respectively. TABLE 12 POD wear rates forspecimens of Example 8 (CIR-SM) Specimen Wear Rate (mg/million cycle)Unirradiated Control 9.78 Irradiated to 2.5 Mrad 9.07 Irradiated to 5Mrad 4.80 Irradiated to 7.5 Mrad 2.53 Irradiated to 10 Mrad 1.54Irradiated to 15 Mrad 0.51 Irradiated to 20 Mrad 0.05 Irradiated to 30Mrad 0.11

TABLE 13 POD wear rates for specimens of Example 10 (WIR-AM) SpecimenWear Rate (mg/million cycle) Unirradiated Control 9.78 Irradiated to 21Mrad 1.15

The results indicate that the cross-linked UHMWPE has far superior wearresistance than the unirradiated control.

D. Gel Content and Swell Ratio

The samples were cut in cubes of size 2×2×2 mm³ and kept submerged inxylene at 130° C. for a period of 24 hours. An antioxidant (1%N-phenyl-2-naphthylamine) was added to the xylene to prevent degradationof the sample. The swell ratio and gel content were calculated bymeasuring the weight of the sample before the experiment, after swellingfor 24 hours and after vacuum drying the swollen sample. The results areshown in Tables 14 and 15 for the specimens obtained from Examples 8 and10. TABLE 14 Gel content and swell ratio for specimens of Example 8(CIR-SM) Specimen Gel Content(%) Swell Ratio Unirradiated Control 89.712.25 Irradiated to 5 Mrad 99.2 4.64 Irradiated to 10 Mrad 99.9 2.48Irradiated to 20 Mrad 99.0 2.12 Irradiated to 30 Mrad 99.9 2.06

TABLE 15 Gel content and swell ratio for specimens of Example 10(WIR-AM) Specimen Gel Content(%) Swell Ratio Unirradiated Control 89.712.25 Irradiated to 21 Mrad 99.9 2.84 Irradiated to 22.5 Mrad 100 2.36

The results show that the swell ratio decreased with increasing absorbeddose indicating an increase in the cross-link density. The gel contentincreased indicating the formation of a cross-linked structure.

Example 12 Free Radical Concentration for UHMWPE Prepared by ColdIrradiation With and Without Subsequent Melting (CIR-SM)

This example illustrates the effect of melting subsequent to coldirradiation of UHMWPE on the free radical concentration. Electronparamagnetic resonance (EPR) was performed at room temperature on thesamples after placing in a nitrogen atmosphere in an air tight quartztube. The instrument used was the Bruker ESP 300 EPR spectrometer andthe tubes used were Taperlok EPR sample tubes (obtained from WilmadGlass Co., Buena, N.J.).

The unirradiated samples did not have any detectable free radicals inthem. During the process of irradiation, free radicals are created whichcan last for at least several years under the appropriate conditions.

The cold-irradiated UHMWPE specimens exhibited a strong free radicalsignal when tested with the EPR technique. When the same samples wereexamined with EPR following a melting cycle, the EPR signal was found tobe reduced to undetectable levels. The absence of free radicals in thecold irradiated subsequently melted (recrystallized) UHMWPE sample meansthat any further oxidative degradation cannot occur via attack onentrapped radicals.

Example 13 Crystallinity and Melting Point at Different Depths forUHMWPE Prepared by Cold Irradiation and Subsequent Melting (CIR-SM)

This example illustrates the crystallinity and melting point atdifferent depths of the cross-linked UHMWPE specimens obtained fromExample 8 with 20 Mrad total radiation dose. Samples were taken atvarious depths from the cross-linked specimen. The crystallinity and themelting point were determined using a Perkin Elmer differential scanningcalorimeter as described in Example 10(B). The results are shown inTable 16. TABLE 16 DSC at a heating rate of 10° C./min for specimen ofExample 8 irradiated to a total dose of 20 Mrad (CIR-SM) Depth (mm)Crystallinity(%) Melting Point(° C.) 0-2 53 137 6-8 54 137  9-11 54 13714-16 34 137 20-22 52 137 26-28 56 137 29-31 52 137 37-40 54 137Unirradiated Control 59 137

The results indicate that the crystallinity varied as a function ofdepth away from the surface. The sudden drop in 16 mm is the consequenceof the cascade effect. The peak in the absorbed dose was located around16 mm where the dose level could be as high as 27 Mrad.

Example 14 Comparison of UHMWPE Prepared by CIR-SM Using Melting in AirVersus Melting Under Vacuum

This example illustrates that the oxidation levels of UHMWPE pucksprepared by CIR-SM, whether melted in air or under vacuum, are the sameas unirradiated pucks at a depth of 3 mm below the surface of the pucks.

Conventional UHMWPE ram extruded bar stock (Hoescht Celanese GUR 415 barstock obtained from Westlake Plastics, Lenni, Pa.) was used. The GUR 415resin used for the bar stock had a molecular weight of 5,000,000 g/moland contained 500 ppm of calcium stearate. The bar stock was cut into“hockey puck” shaped cylinders (height 4 cm, diameter 8.5 cm).

Two pucks were irradiated at room temperature with a dose rate of 2.5Mrad per pass to 17.5 Mrad total absorbed dose as measured on the topsurface (e-beam incidence) (AECL, Pinawa, Manitoba, Canada), with a scanwidth of 30 cm and a conveyor speed of 0.07 cm/sec. The pucks were notpackaged and the irradiation was carried out in air. Subsequent toirradiation, one puck was heated under vacuum to 150° C. for 2 hours,and the other puck was heated in air to 150° C. for 2 hours, so as toattain a state of no detectable residual crystalline content and nodetectable residual free radicals. The pucks were then cooled to roomtemperature at a rate of 5° C./min. The pucks were then analyzed for thedegree of oxidation as described in example 11(A). Table 17 summarizesthe results obtained for the degree of oxidation. TABLE 17 Degree ofoxidation of specimens melted in air versus in vacuum Oxidation Degreeat Post-Melting various depths (A.U.) Specimen Environment 20 μm 100 μm3 mm Unirradiated Control N/A 0.01 0.01 0.02 Irradiated to 17.5 MradVacuum 0.07 0.05 0.02 Irradiated to 17.5 Mrad Air 0.15 0.10 0.01

The results indicated that within 3 mm below the free surfaces theoxidation level in the irradiated UHMWPE specimens dropped to oxidationlevels observed in unirradiated control UHMWPE. This was the caseindependent of post-irradiation melting atmosphere (air or vacuum).Therefore, post-irradiation melting could be done in an air convectionoven without oxidizing the core of the irradiated puck.

Example 15 Method of Making UHMWPE Using Cold Irradiation and SubsequentMelting Using Gamma Irradiation (CIR-SM)

This example, illustrates a method of making UHMWPE that has across-linked structure and has substantially no detectable freeradicals, by cold irradiating with gamma-radiation and then melting theUHMWPE.

Conventional UHMWPE ram extruded bar stock (Hoescht Celanese GUR 415 barstock obtained from Westlake Plastics, Lenni, Pa.) was used. The GUR 415resin used for the bar stock had a molecular weight of 5,000,000 g/moland contained 500 ppm of calcium stearate. The bar stock was cut into“hockey puck” shaped cylinders (height 4 cm, diameter 8.5 cm).

The pucks were irradiated at room temperature at a dose rate of 0.05Mrad/minute to 4 Mrad total absorbed dose as measured on the top surface(gamma ray incidence) (Isomedix, Northboro, Mass.). The pucks were notpackaged and irradiation was carried out in air. Subsequent toirradiation, the pucks were heated to 150° C. under vacuum for 2 hoursso as to melt the polymer and thereby result in the recombination offree radicals leading to substantially no detectable residual freeradicals.

Example 16 I. Method of Making UHMWPE Using Warm Irradiation and PartialAdiabatic Melting with Subsequent Complete Melting (WIR-AM)

This example illustrates a method of making UHMWPE that has across-linked structure, exhibits two distinct melting endotherms in adifferential scanning calorimeter (DSC), and has substantially nodetectable free radicals, by irradiating UHMWPE that has been heated tobelow the melting point so as to generate adiabatic partial melting ofthe UHMWPE and by subsequently melting the UHMWPE.

A GUR 4050 bar stock (made from ram extruded Hoescht Celanese GUR 4050resin obtained from Westlake Plastics, Lenni, Pa.) was machined into 8.5cm diameter and 4 cm thick hockey pucks. Twenty-five pucks, 25 aluminumholders and 25 20 cm×20 cm fiberglass blankets were preheated to 125° C.overnight in an air convection oven. The preheated pucks were eachplaced in a preheated aluminum holder which was covered by a preheatedfiberglass blanket to minimize heat loss to the surroundings duringirradiation. The pucks were then irradiated in air using a 10 MeV, 1 kWelectron beam with a scan width of 30 cm (AECL, Pinawa, Manitoba,Canada). The conveyor speed was 0.07 cm/sec which gave a dose rate of 70kGy per pass. The pucks were irradiated in two passes under the beam toachieve a total absorbed dose of 140 kGy. For the second pass, theconveyor belt motion was reversed as soon as the pucks were out of theelectron beam raster area to avoid any heat loss from the pucks.Following the warm irradiation, 15 pucks were heated to 150° C. for 2hours so as to obtain complete melting of the crystals and substantialelimination of the free radicals.

A. Thermal Properties (DSC) of the specimens prepared in Example 16

A Perkin-Elmer DSC 7 was used with an ice water heat sink and a heatingand cooling rate of 10° C./min with a continuous nitrogen purge. Thecrystallinity of the samples obtained from Example 16 was calculatedfrom the weight of the sample and the heat of melting of polyethylenecrystals (69.2 cal/gm). The temperature corresponding to the peak of theendotherm was taken as the melting point. In the case of multipleendotherm peaks, multiple melting points were reported.

Table 18 shows the variations obtained in the melting behavior andcrystallinity of the polymer as a function of depth away from the e-beamincidence surface. FIG. 8 shows representative DSC melting endothermsobtained at 2 cm below the surface of c-beam incidence obtained bothbefore and after the subsequent melting. TABLE 18 WIR-AM GUR 4050barstock, Total dose = 140 kGy, 75 kGy/pass T 1st peak T 2nd peak T 3rdpeak T 1st peak T 2nd peak Crystallinity Crystallinity after after afterafter after after after irradiation irradiation irradiation subsequentsubsequent irradiation subsequent Depth (mm) (° C.) (° C.) (° C.)melting (° C.) melting (° C.) (%) melting (%) 1.77 109.70 NP 145.10116.35 139.45 53.11 45.26 5.61 118.00 NP 147.80 117.10 141.60 52.6145.46 9.31 113.00 NP 146.40 117.30 141.10 50.13 44.42 13.11 113.47138.07 145.23 116.03 139.83 47.29 43.33 16.89 113.40 137.40 144.80115.90 139.30 47.68 43.05 20.95 113.70 138.33 145.17 115.17 139.63 44.9943.41 24.60 112.40 134.20 143.90 114.90 138.70 49.05 44.40 28.57 112.30NP 145.70 115.90 139.90 50.84 44.40 31.89 111.20 NP 144.50 114.90 138.8051.88 45.28 34.95 NP NP 143.90 112.00 138.45 50.09 45.36 39.02 NP NP139.65 114.95 138.30 49.13 46.03*NP: The peak is not present

These results indicate that the melting behavior of UHMWPE changesdrastically after the subsequent melting step in this embodiment of theWIR-AM process. Before the subsequent melting, the polymer exhibitedthree melting peaks, while after subsequent melting it exhibited twomelting peaks.

B. Electron Paramagnetic Resonance (EPR) of the Specimens Prepared inExample 16

EPR was performed at room temperature on samples obtained from Example16 after placing the samples in an air tight quartz tube in a nitrogenatmosphere. The instrument used was the Bruker ESP 300 EPR spectrometerand the tubes uses were Taperlok EPR sample tubes (obtained from WilmadGlass Co., Buena, N.J.).

The unirradiated samples did not have any detectable free radicals inthem. During the process of irradiation, free radicals are created whichcan last for at least several years under the appropriate conditions.

Before the subsequent melting, the EPR results showed a complex freeradical peak composed of both peroxy and primary free radicals. Afterthe subsequent melting the EPR free radical signal was reduced toundetectable levels. These results indicated that the free radicalsinduced by the irradiation process were substantially eliminated afterthe subsequent melting step. Thus, the UHMWPE was highly resistant tooxidation.

Example 17 II. Method of Making UHMWPE Using Warm Irradiation andPartial Adiabatic Melting with Subsequent Complete Melting (WIR-AM)

This example illustrates a method of making UHMWPE that has across-linked structure, exhibits two distinct melting endotherms in DSC,and has substantially no detectable free radicals, by irradiating UHMWPEthat has been heated to below the melting point so as to generate theadiabatic partial melting of the UHMWPE and by subsequently melting theUHMWPE.

A CUR 4020 bar stock (made from ram extruded Hoescht Celanese GUR 4020resin obtained from Westlake Plastics, Lenni, Pa.) was machined into 8.5cm diameter and 4 cm thick hockey pucks. Twenty-five pucks, 25 aluminumholders and 25 20 cm×20 cm fiberglass blankets were preheated to 125° C.overnight in an air convection oven. The preheated pucks were eachplaced in a preheated aluminum holder which was covered by a preheatedfiberglass blanket to minimize heat loss to the surroundings duringirradiation. The pucks were then irradiated in air using a 10 MeV, 1 kWelectron beam with a scan width of 30 cm (AECL, Pinawa, Manitoba,Canada). The conveyor speed was 0.07 cm/sec which gave a dose rate of 70kGy per pass. The pucks were irradiated in two passes under the beam toachieve a total absorbed dose of 140 kGy. For the second pass, theconveyor belt motion was reversed as soon as the pucks were out of theelectron beam raster area to avoid any heat loss from the pucks.Following the warm irradiation, 15 pucks were heated to 150° C. for 2hours so as to obtain complete melting of the crystals and substantialelimination of the free radicals.

Example 18 III. Method of Making UHMWPE Using Warm Irradiation andPartial Adiabatic Melting with Subsequent Complete Melting (WIR-AM)

This example illustrates a method of making UHMWPE that has across-linked structure, exhibits two distinct melting endotherms in DSC,and has substantially no detectable free radicals, by irradiating UHMWPEthat has been heated to below the melting point so as to generateadiabatic partial melting of the UHMWPE and by subsequently melting theUHMWPE.

A GUR 1050 bar stock (made from ram-extruded Hoescht Celanese GUR 1050resin obtained from Westlake Plastics, Lenni, Pa.) was machined into 8.5cm diameter and 4 cm thick hockey pucks. Eighteen pucks, 18 aluminumholders and 18 20 cm×20 cm fiberglass blankets were preheated to 125°C., 90° C., or 70° C., in an air convection oven overnight. Six puckswere used for each different pre-heat temperature. The preheated puckswere each placed in a preheated aluminum holder which was covered by apreheated fiberglass blanket to minimize heat loss to the surroundingsduring irradiation. The pucks were then irradiated in air using a 10 MeVand 1 kW electron beam with a scan width of 30 cm (AECL, Pinawa,Manitoba, Canada). The conveyer speed was 0.06 cm/sec which gave a doserate of 75 kGy per pass. The pucks were irradiated in two passes underthe beam to accumulate a total of 150 kGy of absorbed dose. For thesecond pass, the conveyor belt motion was reversed as soon as the puckswere out of the electron beam raster area to avoid any heat loss fromthe pucks. Following the warm irradiation, half of the pucks were heatedto 150° C. for 2 hours so as to obtain complete melting of the crystalsand substantial elimination of the free radicals.

A. Thermal Properties of the Specimens Prepared in Example 18

A Perkin-Elmer DSC 7 was used with an ice water heat sink and a heatingand cooling rate of 10° C./min with a continuous nitrogen purge. Thecrystallinity of the samples obtained from Example 18 was calculatedfrom the weight of the sample and the heat of melting of polyethylenecrystals (69.2 cal/gm). The temperature corresponding to the peak of theendotherm was taken as the melting point. In the case of multipleendotherm peaks, multiple melting points were reported.

Table 19 shows the effect of pre-heat temperature on the meltingbehavior and crystallinity of the polymer. FIG. 9 shows the DSC profileof a puck processed with the WIR-AM method at a pre-heat temperature of125° C. both before and after subsequent melting. TABLE 19 WIR-AM GUR1050 barstock, Total dose = 150 kGy, 75 kGy/pass T 1st peak T 2nd peak T1st peak T 2nd peak T 3rd peak after after Crystallinity Crystallinityafter after after subsequent subsequent after after Preheat irradiationirradiation irradiation melting melting irradiation subsequent (° C.) (°C.) (° C.) (° C.) (° C.) (° C.) (%) melting (%) 125 114.6 135.70 143.5114.85 135.60 42.81 40.85 90 NP 142.85 NP 116.75 136.95 52.39 44.31 70NP 141.85 NP NP 136.80 51.59 44.62*NP: The peak is not present

These results indicate that the melting behavior of UHMWPE changesdrastically after the subsequent melting step in this embodiment of theWIR-AM process. Before the subsequent melting, the polymer exhibitedthree melting peaks, while after subsequent melting it exhibited twomelting peaks.

Example 19 IV. Method of Making UHMWPE Using Warm Irradiation andPartial Adiabatic Melting with Subsequent Complete Melting (WIR-AM)

This example illustrates a method of making UHMWPE that has across-linked structure, exhibits two distinct melting endotherms in DSC,and has substantially no detectable free radicals, by irradiating UHMWPEthat has been heated to below the melting point so as to generateadiabatic partial melting of the UHMWPE and by subsequently melting thepolymer.

A GUR 1020 bar stock (made from ram extruded Hoescht Celanese GUR 1020resin obtained from Westlake Plastics, Lenni, Pa.) was machined in 7.5cm diameter and 4 cm thick hockey pucks. Ten pucks, 10 aluminum holdersand 10 20 cm×20 cm fiberglass blankets were preheated to 125° C.overnight in an air convection oven. The preheated pucks were eachplaced in a preheated aluminum holder which was covered by a preheatedfiberglass blanket to minimize heat loss to the surroundings duringirradiation. The pucks were then irradiated in air using a 10 MeV, 1 kWlinear electron beam accelerator (AECL, Pinawa, Manitoba, Canada). Thescan width and the conveyor speed was adjusted to achieve the desireddose rate per pass. The pucks were then irradiated to 61, 70, 80, 100,140, and 160 kGy of total absorbed dose. For 61, 70, 80 kGy absorbeddose, the irradiation was completed in one pass; while for 100, 140, and160 it was completed in two passes. For each absorbed dose level, sixpucks were irradiated. During the two pass experiments, for the secondpass, the conveyor belt motion was reversed as soon as the pucks wereout of the electron beam raster area to avoid any heat loss from thepucks. Following the irradiation, half of the pucks were heated to 150°C. for 2 hours in an air convection oven so as to obtain completemelting of the crystals and substantial elimination of the freeradicals.

Example 20 V. Method of Making UHMWPE Using Warm Irradiation and PartialAdiabatic Melting with Subsequent Complete Melting (WIR-AM)

This example illustrates a method of making UHMWPE that has across-linked structure, exhibits two distinct melting endotherms in DSC,and has substantially no detectable free radicals, by irradiating UHMWPEthat has been heated to below the melting point so as to generateadiabatic partial melting of the UHMWPE and by subsequently melting thepolymer.

A GUR 4150 bar stock (made from ram extruded Hoescht Celanese GUR 4150resin obtained from Westlake Plastics, Lenni, Pa.) was machined into 7.5cm diameter and 4 cm thick hockey pucks. Ten pucks, 10 aluminum holdersand 10 20 cm×20 cm fiberglass blankets were preheated to 125° C.overnight in an air convection oven. The preheated pucks were eachplaced in a preheated aluminum holder which was covered by a preheatedfiberglass blanket to minimize heat loss to the surroundings duringirradiation. The pucks were then irradiated in air using a 10 MeV, 1 kWlinear electron beam accelerator (AECL, Pinawa, Manitoba, Canada). Thescan width and the conveyor speed was adjusted to achieve the desireddose rate per pass. The pucks were irradiated to 61, 70, 80, 100, 140,and 160 kGy of total absorbed dose. For each absorbed dose level, sixpucks were irradiated. For 61, 70, 80 kGy absorbed dose, the irradiationwas completed in one pass; for 100, 140 and 160 kGy, it was completed intwo passes.

Following the irradiation, three pucks out of each different absorbeddose level were heated to 150° C. for 2 hours to completely melt thecrystals and reduce the concentration of free radicals to undetectablelevels.

A. Properties of the Specimens Prepared in Example 20

A Perkin-Elmer DSC 7 was used with an ice water heat sink and a heatingand cooling rate of 10° C. per minute with a continuous nitrogen purge.The crystallinity of the samples obtained from Example 20 was calculatedfrom the weight of the sample and the heat of melting of polyethylenecrystals (69.2 cal/gm). The temperature corresponding to the peak of theendotherm was taken as the melting point. In the case of multipleendotherm peaks, multiple melting points were reported.

The results obtained are shown in Table 20 as a function of totalabsorbed dose level. They indicate that crystallinity decreases withincreasing dose level. At the absorbed dose levels studied, the polymerexhibited two melting peaks (T₁=−118° C., T₂=−137° C.) after thesubsequent melting step. TABLE 20 WIR-AM GUR 4150 barstock T 1st peak T2nd peak T 3rd peak T 1st peak T 2nd peak Crystallinity Crystallinityafter after after after after after after Irradiation irradiationirradiation irradiation subsequent subsequent irradiation subsequentdose (kGy) (° C.) (° C.) (° C.) melting (° C.) melting (° C.) (%)melting (%) 160 113.4 135.10 143.20 114 135.90 41.97 39.58 140 114.6135.10 143.60 116.2 138.60 45.25 41.51 100 118.7 125.10 143.50 118.2138.20 47.18 42.58 80 115.7 NP 142.00 119.1 137.60 50.61 44.52 70 114.8NP 141.40 118.9 137.00 52.36 44.95 61 114.6 NP 140.20 119.1 136.00 53.0145.04*NP: The peak is not present

Example 21 Temperature Rise During WIR-AM Process

This example demonstrates that the temperature rises during the warmirradiation process leading to adiabatic partial or complete melting ofthe UHMWPE.

A GUR 4150 bar stock (made from ram extruded Hoescht Celanese GUR 4150resin obtained from Westlake Plastics, Lenni, Pa.) was machined into a8.5 cm diameter and 4 cm thick hockey puck. One hole was drilled intothe body-center of the puck. A Type K thermocouple was placed in thishole. The puck was pre-heated to 130° C. in air convection oven. Thepuck was then irradiated using 10 MeV, 1 kW electron beam (AECL, Pinawa,Manitoba, Canada). The irradiation was carried out in air with a scanwidth of 30 cm. The dose rate was 27 kGy/min and the puck was leftstationary under the beam. The temperature of the puck was constantlymeasured during irradiation.

FIG. 11 shows the temperature rise in the puck obtained during theirradiation process. Initially, the temperature is at the pre-heattemperature (130° C.). As soon as the beam is turned on, the temperatureincreases, during which time the UHMWPE crystals melt. There is meltingof smaller size crystals starting from 130° C., indicating that partialmelting occurs during the heating. At around 145° C. where there is anabrupt change in the heating behavior, complete melting is achieved.After that point, temperature continues to rise in the molten material.

This example demonstrates that during the WIR-AM process, the absorbeddose level (duration of irradiation) can be adjusted to either partiallyor completely melt the polymer. In the former case, the melting can becompleted with an additional melting step in an oven to eliminate thefree radicals.

Example 22 Method of Making UHMWPE Using Cold Irradiation and AdiabaticHeating with Subsequent Complete Melting (CIR-AM)

This example illustrates a method of making UHMWPE that has across-linked structure, and has substantially no detectable freeradicals, by irradiating UHMWPE at a high enough dose rate to generateadiabatic heating of the UHMWPE and by subsequently melting the polymer.

A GUR 4150 bar stock (made from ram extruded Hoescht Celanese GUR 4150resin obtained from Westlake Plastics, Lenni, Pa.) was machined into 8.5cm diameter and 4 cm thick hockey pucks. Twelve pucks were irradiatedstationary, in air, at a dose rate of 60 kGy/min using 10 MeV, 30 kWelectrons (E-Beam Services, Cranbury, N.J.). Six of the pucks wereirradiated to a total dose of 170 kGy, while the other six wereirradiated to a total dose of 200 kGy. At the end of the irradiation thetemperature of the pucks was greater than 100° C.

Following the irradiation, one puck of each series was heated to 150° C.for 2 hours to melt all the crystals and reduce the concentration offree radicals to undetectable levels.

A. Thermal Properties of the Specimens Prepared in Example 22

A Perkin-Elmer DSC 7 was used with an ice water heat sink and a heatingand cooling rate of 10° C. per minute with a continuous nitrogen purge.The crystallinity of the samples obtained from Example 22 was calculatedfrom the weight of the sample and the heat of melting of polyethylenecrystals (69.2 cal/gm). The temperature corresponding to the peak of theendotherm was taken as the melting point.

Table 21 summarizes the effect of total absorbed dose on the thermalproperties of CIR-AM UHMWPE both before and after the subsequent meltingprocess. The results obtained indicate one single melting peak bothbefore and after the subsequent melting step. TABLE 21 CIR-AM GUR 4150barstock Crystallinity Crystallinity T peak after T peak after afterafter Irradiation irradiation subsequent irradiation subsequent dose(kGy) (° C.) melting (° C.) (%) melting (%) 170 143.67 137.07 58.2545.27 200 143.83 136.73 54.74 43.28

Example 23 Comparison of Tensile Deformation Behavior of UnirradiatedUHMWPE. Cold-Irradiated and Subsequently Melted UHMWPE (CIR-SM), andWarm Irradiated and Partially Adiabatic Melted and Subsequently MeltedUHMWPE (WIR-AM)

This example compares the tensile deformation behavior of UHMWPE in itsunirradiated form, and irradiated forms via CIR-SM and WIR-AM methods.

The ASTM D638 Type V standard was used to prepare dog bone specimens forthe tensile test. The tensile test was carried out on an Instron 4120Universal Tester at a cross-head speed of 10 mm/min. The engineeringstress-strain behavior was calculated from the load-displacement datafollowing ASTM D638.

The dog bone specimens were machined from GUR 4150 hockey pucks (madefrom ram extruded Hoescht Celanese GUR 4150 resin obtained from WestlakePlastics, Lenni, Pa.) that were treated by CIR-SM and WIR-AM methods.For the CIR-SM, the method described in Example 8 was followed, whilefor WIR-AM, the method described in Example 17 was followed. In bothcases, the total dose administered was 150 kGy.

FIG. 12 shows the tensile behavior obtained for the unirradiatedcontrol, CIR-SM treated, and WIR-AM treated specimens. It shows thevariation in tensile deformation behavior in CIR-SM and WIR-AM treatedUHMWPE, even though in both methods the irradiation was carried out to150 kGy.

Those skilled in the art will be able to ascertain using no more thanroutine experimentation, many equivalents of the specific embodiments ofthe invention described herein. These and all other equivalents areintended to be encompassed by the following claims.

1. A medical prosthesis for use within the body, said prosthesis beingformed of radiation treated ultra high molecular weight polyethylenehaving substantially no detectable free radicals. 2-5. (canceled)
 6. Theprosthesis of claim 1, wherein said ultra high molecular weightpolyethylene has three melting peaks.
 7. The prosthesis of claim 1,wherein said ultra high molecular weight polyethylene has two meltingpeaks. 8-123. (canceled)
 124. A preformed material for subsequentproduction of a medical implant with improved wear resistance comprisinga polyethylene cross-linked at least twice by irradiation and thermallytreated by annealing after each irradiation.
 125. The preformed materialas set forth in claim 124, wherein the material is cross-linked by atotal radiation dose from about 2 to about 100 MRad.
 126. The preformedmaterial as set forth in claim 125, wherein the total radiation dose isbetween about 5 to about 10 MRad.
 127. The preformed material as setforth in claim 126, wherein three radiation doses are applied with anincremental dose for each irradiation is between about 2 and about 5MRad.
 128. The preformed material as set forth in claim 124, whereinthree radiation doses are applied with an incremental dose for eachirradiation being between about 2 and about 5 MRad.
 129. The preformedmaterial as set forth in claim 128, wherein the total radiation dose isbetween about 5 to about 10 MRad.
 130. The orthopedic preformed materialas set forth in claim 124, wherein the polyethylene has a weight averagemolecular weight of greater than 400,000.
 131. The preformed material asset forth in claim 124, wherein the annealing takes place in air at atemperature greater than 25° C.
 132. The preformed material as set forthin claim 131, wherein the annealing takes place for a time andtemperature selected to be at least equivalent to heating saidirradiated material at 50° C. for 144 hours.
 133. The preformed materialas set forth in claim 132, wherein said material is heated for at leastabout four hours.
 134. The orthopedic preformed material as set forth inclaim 124, wherein the polyethylene is at room temperature for eachirradiation.
 135. The preformed material as set forth in claim 124,wherein the polyethylene is cross-linked three times by irradiation andthermally treated by annealing after each irradiation at a temperaturebetween 25° C. and 135° C. for at least 4 hours.
 136. A method forincreasing the wear resistance of a preformed polyethylene comprising:irradiating the preformed polyethylene in the solid state at least twotimes; and annealing the preformed polyethylene after each irradiation.137. The method for increasing the wear resistance as set forth in claim136, wherein the material is cross-linked by a total radiation dose fromabout 1 to about 100 MRad.
 138. The method for increasing the wearresistance as set forth in claim 137, wherein the total radiation doseis between about 5 to about 10 MRad.
 139. The method for increasing thewear resistance as set forth in claim 138, wherein an incremental dosefor each irradiation is between about 2 and about 5 MRad.
 140. Themethod for increasing the wear resistance as set forth in claim 139,wherein an incremental dose for each irradiation is between about 2 andabout 5 MRad.
 141. The method for increasing the wear resistance as setforth in claim 140, wherein the total radiation dose is between about 4to about 10.5 MRad.
 142. The method for increasing the wear resistanceas set forth in claim 141, wherein the weight average molecular weightof the polyethylene is greater than 400,000.
 143. The method forincreasing the wear resistance as set forth in claim 142, wherein theannealing takes place at a temperature greater than 25° C.
 144. Themethod as set forth in claim 136 wherein the annealing takes placebetween 110° C. and 135° C.
 145. The method for increasing the wearresistance as set forth in claim 144, wherein the annealing takes placefor a time and temperature selected to be at least equivalent to heatingsaid irradiated material at 50° C. for 144 hours.
 146. The method forincreasing the wear resistance as set forth in claim 136, wherein saidmaterial is heated for at least about 4 hours.
 147. The method as setforth in claim 136 further including the step of machining the preformedpolyethylene into a medical implant.
 148. The method as set forth inclaim 136, wherein the material polyethylene is cross-linked three timesby irradiation and thermally treated by annealing after each irradiationat a temperature between 25° C. and 135° C. for at least about 4 hours.149. A medical device comprising a polyethylene material irradiated atleast two times and annealed at a temperature lower than the meltingpoint of the material after each irradiation.